1. GOCE OBSERVATIONS FOR DETECTING UNKNOWN TECTONIC FEATURES BRAITENBERG C. (1), MARIANI P. (1), REGUZZONI M. (2), USSAMI N. (3) (1)Department of Geosciences, University of Trieste, Trieste ( ITALY), (2) Geophysics of the Lithosphere Department - OGS, c/o Politecnico di Milano - Polo Regionale di Como, Como, Italy (3)Departamento de Geofisica, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, Brasil Home page: http://www2.units.it/~braitenberg/http://www2.units.it/~braitenberg/ e-mail: berg@units.it

2. Goal Locate density changes in Earth’s crust Crustal parameters necessary for: – Exploration purposes – Seismic risk estimation – Volcanic risk estimation Remote and unaccessible areas: superficial properties known gravity study useful geophysical means of investigation

3. TOPIC Sensitivity analysis of GOCE for tectonic structures Model: spherical shell of variable density or thickness Input: simulated GOCE degree error curve Rms error of tensor components at satellite height Error curves of existing gravity field models (EGM2008)

4. DENSITY AND TECTONICS GOCE measures gravity and gravity gradient -> sensitive to tectonic structures with density changes. -> structures without density change are transparent -> GOCE only: upper limit of degree N=200; tectonic structures greater than  min= 100 km

5. PREM Earth model (Anderson, 1989) Earth Density

6. Lama & Vutukuri, 1978.

7. Spherical shell model Spherical shell model: mass layer expanded in spherical harmonics Gravity models in spherical harmonic expansion

8. Shell model for sensitivity analysis – Harmonic expansion of sheet: –Mass model: sheet mass with average radius R

9. Anomalous potential and derived quantities Potential Gravity Gravity gradient R: shell radius r: calculation point

10. Resolution power for geological structures Degree error variance: corresponds to smallest detectable field generated by mass source Invert for smallest dectable sheet mass At density discontinuities : mass layer interpreted as oscillation of boundary Boundary oscillation:

11. Gravity anomaly cumulative and single degree error 55km200km100km λ/2= GOCE error curve:. Dr. Mirko Reguzzoni, POLIMI & OGS

12. Invert degree error curves Mass-Layer: Crust-Mantle discontinuity We set: average depth (30 to 70 km) and density contrast across boundary (500 kg/m 3 ) We find: minimum decetable oscillation amplitude of boundary.

13. Minimum detectable Moho undulation amplitude Single degree error curves

14. GOCE improvement Up to one order of magnitude improvement for degree range 52 to 200 Average depth important. Greater depth with reduced resolution Depth depends on geodynamic context: Craton (45 km), High topography (up to 70 km), normal crust: 35 km

15. Basement resolution Mass layer represents basement - sediment transition Average depth 0 km to 10 km Density contrast: greatly variable Sediments follow exponential density increase due to compaction

16. Basement resolution

17. GOCE resolution Single degree error curves give meter level resolution Basement depth not important Density contrast predominant effect

18. GOCE Gradient measurements Use tensor components at satellite height Infer crustal density variations Question: how does sensitivity compare to sensitivity of airborne gravity?

19. Observation error levels GOCE GOCE root mean square error of data along orbit (after processing) Diagonal tensor elements [mE] Along trackAcross trackRadial T ξξ T ηη T rr 1104 (Migliaccio et al., 2008)

20. Rms error airborne gravity (Van Kann, 2004)

21. Lower crust density sensitivity Model: layer 10 km thick above Moho (35 km depth) Trr observed at satellite height – rms: 0.1 mE to 100 mE dg observed at 1000 m height – rms: 0.01 mgal to 10 mgal

23. Sensitivity density lower crust rms of 1 mgal at 1000m has comparable sensitivity with 1mE rms at satellite height (at wavelengths of 170 km) GOCE sensitivity competes with aerogravity surveys Sensitivity for GOCE better at longer wavelengths

24. Example Tibetan crust Terrestrial data are scarce and lacking in Himalaya Tibetan plateau and Tarim basin contain spectral components accessible to GOCE Further investigation is needed of crustal densities

25. Tibetan Moho (Braitenberg et al., 2003; Shin et al., 2009)

26. Power spectrum Tibetan Moho (Shin et al., 2009)

27. Conclusions GOCE expected to contribute improvement to: – Crustal density structure for wavelengths between 900 km and 220 km. – In particular: crustal thickness variations and basement undulations – Crustal densities – 1 mE at satellite height retrieves as 1 mgal airborne – Advantage: truly global